Development and Care of the Young 171 



are treated with the greatest care. When broken open they 

 are either eaten or carefully removed by all birds which possess 

 the cleaning instinct, an illustration of which we have already 

 seen in the Common Tern. (See p. 33.) 



The Herring Gull upon entering her nest usually turns her 

 eggs with her bill, and if you change their position she will in- 

 variably turn them 

 again. The domestic 

 hen stirs up her eggs 

 with her feet. There 

 is something more 

 than the comfort of 

 the sitting bird in- 

 volved in such acts, 

 for the eggs need 

 damping, and by this 

 means each side of the 

 egg is exposed to the 

 moisture of the ground 

 or bottom of the nest. 

 Then, in a full nest, 

 constant rolling tends 

 to equalize the distri- 

 bution of warmth, 

 bringing all the eggs Fig io3 Photomicrograph of the chick in the egg) 



Successively into the at the thirty-third hour of incubation. The vesicles 



j_ 1 /pi of the eyes and brain, the delicate, tubular heart, and 



LCG - 3 the primitive segments of the body are plainly seen. 



turning is even more Embryo, one-fifth inchlong; enlarged nearly ten times. 



useful in preventing 



the germ or embryo from sticking to the shell, a necessity recog- 

 nized by the breeder of domestic fowls, who twice daily turns 

 the eggs in his incubators. 



The embryo appears first as a thin disc on the surface of the 

 yolk, over which it gradually spreads until the whole is enclosed 

 as within a sac. It always lies uppermost, next to the warm 

 breast of the mother, for in the first place the egg is lightest at 

 its growing pole, and in the second the spherical mass moves 



